Update python-markdown

[giginet]

I'd like to run pokedex on macOS with Apple Silicon. However, the old python-markdown doesn't support building for M1 chips.

I updated python-markdown for the latest version.

• markdown.util.etree is deprecated on 3.2. According to the official documentation, use xml.etree.ElementTree instead. https://python-markdown.github.io/change_log/release-3.2/#markdownutiletree-deprecated
• latest python-markdown doesn't support ancient Pythons. So I dropped 2.7 support. Python 2.x is no longer supported officially.

[Naramsim]

Hi, can you point this PR to https://github.com/veekun/pokedex ?

[giginet]

I can submit PR to the original repo. However, it seems to be no more maintained....

[giginet]

I submitted veekun/pokedex#369

[Naramsim]

Let's merge it here then!